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Jim, Stefan suggested you might be the best person to ask about this?

Yes, it is a known bug.

Okay, is the bug in the documentation or the functionality?

Take your pick. The feature is mainly for backward compatibility.
I'd be happy to drop it, especially since I didn't catch
this before the 2.9/3.2 releases, which I deeply regret.
(Those releases were, sadly, such a scramble. I wish I hadn't
spent so much time trying to improve the Twisted integration.)
Dropping the feature would make this a documentation bug. :)